1. Field of Invention
This invention relates to radial tires and more particularly to radial tires for use on heavy load vehicles such as trucks and buses.
2. Prior Art
In the prior art, radial tires generally have a carcass ply layer whose cords are radially disposed at an angle approximately 90.degree. relative to the circumference of the tire and a steel belt layer which is provided as a reinforcing layer around the outside of the carcass ply layer. However, radial tires used on trucks and buses are used at a high air pressure. As a result, a large amount of stress is applied to the steel belt layer causing strain in a direction which decreases the angle of disposition of the steel cord of the belt layer. Accordingly, the outer diameter of the tire tends to grow. Furthermore, when the carcass cords are elongated by the tensile force generated in the carcass layer, the cross-sectional width of the tire also grows so that the cross-sectional shape of the tire is generally changed from the shape of the tire when the tire was new.
If high-modulus steel cords are used in the carcass plies, there is little elongation of the carcass cords. As a result, there is almost no growth in the width of the tire and the cross-sectional shape of tire shows a sizeable growth in the direction of the outer diameter of the tire, as indicated by the dotted line in FIG. 1. On the other hand, if low-modulus cords are used in the carcass plies the growth of the cross-sectional width W of the tire is greatly increased, so that the cross-sectional shape of the tire grows in the direction of the width of the tire, as indicated by the dotted line in FIG. 2.
When the cross-sectional shape of the tire undergoes such a great change, a shearing force is concentrated in the vicinity of the beginning of contact CB between the belt layer B and the carcass layer C as is shown in FIG. 3. As a result, separation occurs therein after tire running and the strength retention coefficient of the carcass ply cords drops remarkably in the beginning of the contact CB. Accordingly, the useful life of the tire is shortened. Thus, tire growth in the direction of the outer diameter of the tire and in the direction of the width of the tire has a substantial effect on tire durability.
If low-modulus steel cords are used for the cords in the carcass layer, it is necessary to increase the number of plies in the carcass layer in order to prevent growth of the tire in the direction of the cross-sectional width of the tire, such as is shown in FIG. 2. However, this leads to a drawback in that the carcass layer becomes excessively strong and also drawbacks in an increase of the tire weight and in heat build-up during tire running.
On the other hand, if high modulus steel cords are used for the cords in the carcass layer, it is necessary to increase the modulus of the belt layer in order to prevent growth of the tire in the direction of the outer diameter of the tire such as is shown in FIG. 1. However, any increase in the number of plies in the belt layer for this purpose is disadvantageous in the increase of tire weight and heat build-up during the tire running.
Accordingly, in conventional radial tires used on trucks and buses, for an example, a belt layer comprises three plies wherein the outer two plies have cords disposed at a relatively small angle, i.e. 15.degree. to 25.degree., and the ply installed on the carcass layer side has cords disposed at a relatively large angle, i.e. 45.degree. to 75.degree.. In this way, triangle elements which undergo little change are formed at the area that cords of the respective plies intersect, so that the growth of the belt layer is restrained and separation in the vicinity of the beginning of contact CB between the belt layer and carcass layer caused by tire growth is prevented by the buffer effect produced by the intermediate angle ply installed between the small angle plies and the carcass layer disposed at an angle of 90.degree.. FIG. 4 illustrates the disposition of the cords in such a conventional belt layer. Counting from the carcass layer side, B1 indicates the cords of the first ply, B2 indicates the cords of the second ply and B3 indicates the cords of the third ply of the belt layer. The cords B2 of the second ply intersect with the cords B3 of the third ply and both cords B2 and B3 are disposed at angles of 15.degree. to 25.degree. relative to the circumferential line CL of the tire. Meanwhile, the cords B1 of the first ply are disposed at an angle of 45.degree. to 75.degree. relative to the circumferential line CL of the tire. Thus, the three plies form triangle elements as shown by XYZ , X'Y'Z' in FIG. 4. However, since there is a large difference of angle between the cords of the first ply disposed at 45.degree. to 75.degree. and the cords of the other plies, the manufacturing process is complicated. Furthermore, the first ply having such a large angle is not particularly useful for restraining the stress caused in the circumferential direction of tire and the substantial strength of the steel cords cannot be efficiently utilized. Furthermore, since a multiple number of plies are formed using steel cords, the weight of the tire is substantially increased.